Diapers and even pull ups training pants do delay potty training.
When potty training a child who is wearing diapers while potty training they continue to associate the diaper with peeing and pooping.
The diaper also absorbs the pee so that the child does not feel wet.
When potty training they should be in underwear so that when they pee they feel the pee and wetness which is uncomfortable in underwear.
In a diaper the wet diaper is not uncomfortable.
Also having poop in underwear is not a good feeling although it's not as bad in a diaper.
Still though if the child needs a diaper at night or when out and about to prevent them from getting their pants wet then they should wear the diaper.
However at home when potty training they should wear underwear.
The age at which kids should stop using diapers is by age 3 at the latest.
However some kids may take a bit longer to potty train and some kids may not be ready to potty train and stop wearing diapers until age 4 years old.
Kids usually potty train between the ages of 18 months to 3 years old which is the most normal age range for potty training.
But every kid is different and every kid will potty train at different ages.
Personally I believe that when parents say their kid is potty trained at 18 months to 2 years of age the kid is actually not potty trained.
At 18 months to 2 years of age parents have to really take kids that age to the toilet themselves and remind them to potty on the toilet.
That is actually the parents being potty trained and not the kid.
Most often when you do begin potty training by age 18 months to 2 years old the kid is not fully potty trained until 3 years old or a little over 3.
If parents would just wait until age 3 years old to potty train then they would save a lot of work and trouble of having to clean underwear, floors etc.
There's actually no age limit for wearing diapers.
Anyone of any age should wear diapers if they need them.
Some kids may need to wear a diaper at night for bed wetting until age 8 to 10 years old and some continue to wet the bed until 17 years old.
Also some kids may be urinary incontinent and need to wear a diaper all the time even when they are potty trained.
So the kid should wear diapers until they no longer need to wear them.